Boxer can only emulate MS-DOS games, and will warn you if you try to import a game made for Mac OS.
Mac OS X will no longer run games that were written for Mac OS 9 and earlier (“Classic Mac OS”). If OS X won’t run your game, and you can’t find a DOS version of the game for Boxer to play with, then you’ll need a Classic Mac OS emulator instead:
Each of these emulators requires you to obtain a suitable Macintosh ROM image from elsewhere and to install an actual copy of Mac OS into the emulator yourself. Good luck with that!
Some DOS games come on “hybrid” CD-ROMs that contain both Mac and MS-DOS versions of the game: each on separate halves of the disc. Only the Mac half of the disc is visible in OS X: because Boxer cannot access the other half with the DOS version on it, Boxer cannot import DOS games from hybrid CDs.
Instead, you could: